Roy Schilling, BP America
Jacky Massaglia, V&M Tubes
The next generation of development projects in US Gulf of Mexico deepwater will involve high pressure/high temperature (HP/HT) requirements plus sour service compliant materials. Over the last 10 years, dry tree concepts have been prevalent and the use of high strength steels (HSS) with threaded and coupled (T&C) riser connectors have enabled those concepts to be technically feasible, lighter, and cost effective in water depths up to 10,000 ft (3,048 m).
Such critical requirements have resulted in several studies on the design and qualification of heavy wall riser connectors for applications in TTR and/or SCR systems. In addition, fundamental material and testing efforts are also being made to assess and qualify the fatigue resistance of heavy wall pipes made of HSS up to 125 ksi in sour service.
Next generation dry trees
Dry tree concepts offer advantages in terms of drilling and completion costs savings and long-term-well maintenance and intervention capability. Historically, those systems have used forged and machined weld-on premium riser connectors that require steel in grades of X-80 ksi and less to achieve the weld ductility necessary for the riser fatigue design. In many cases, the welds have also been ground “essentially flush” (1/32 in. or 0.8 mm) both externally and internally with strict procedural requirements, special equipment, and exacting surface finishes (i.e. RMS<200) to meet the performance requirements of long-term operations in areas of significant loop currents. TheHorn Mountain truss spar set a world record for these types of riser systems in 2002 in about 5,450 ft (1,661 m) water depth and a pressure rating of 5,500 psi.
BP has looked at dry trees for HP/HT developments in the GoM, but the X-80 riser designs became so heavy they were not considered workable. BP installed theHolstein truss spar in 2005, which was the first dry tree spar application of hydro-pneumatic tensioned risers, with both X-80 weld-on connectors and T95 T&C riser connectors. The tensioner concept was so successful that air cans became the secondary option, and tensioners the default/preferred solution. At the same time, BP was looking to the next generation of developments in water depths out to 10,000 ft (3,048 m). In these extreme water depths, the X-80 riser design was not cost effective, even with larger hydraulic tensioners. Switching to higher strength steel and a T&C connector design, however, made the systems much lighter and became an enabling technology for dry trees.
Fatigue crack initiation site.
Current discoveries in the GoM combine these extreme water depths with HP/HT reservoir conditions where mudline shut in pressures can approach or even exceed 15 ksi. Those critical conditions lead to the limits of current drilling and completion design, and it is now required to go beyond and look for alternative solutions that will provide additional technical capability. From an economical standpoint, the longer times needed to drill and complete such wells combined with ever increasing MODU rig day rates has put greater emphasis on dry tree technology as an enabler to keep these developments possible.
As part of its GoM HP/HT technology program, BP initiated the design and qualification of a 15K dual-barrier dry tree riser system capable of installation in water depths up to 10,000 ft. The outer riser is 16 in. (40.5 cm) OD with 1 in. (2.5 cm) wall thickness Q-125 material and rated to ≥10,000 psi to act as the dual barrier during completion and well workovers operations. The internal drift of this outer riser provides the option of running 135⁄8 in. (34.6 cm) nested subsea wellhead hangers for the production casing if desired. The inner riser was sized at 113⁄4 in. (30 cm) OD, with 1.1 in. (2.8 cm) wall thickness C-110 material and rated to ≥15,000 psi, to act as the primary containment of the dual barrier system during completion and production operations. The top tension required to support these systems was about 2,000 kips in 4,500 ft (1,372 m) water depth and about 3,300 kips in 8,000 ft (2,438 m) well within hydro-pneumatic tensioner capability.
Fatigue life improvement demonstration.
While trial casing runs have been delivered in these sizes, T&C riser connectors do not exist and need to be designed and qualified for the fatigue service and high pressures applicable to deepwater riser systems. In the meantime, fatigue resistance of high strength steels also needs to be assessed and qualified in various environments including sea water and H2S/CO2.
New generation connectors
The first step in the fatigue oriented design was to improve the fatigue performance of an existing downhole casing connector, keeping the same basic concepts but introducing fatigue enhancements.
A series of full-scale fatigue failure tests were first performed on a number of samples to identify the fatigue failure mechanism of the connector. Cracks always occurred at the root of the last engaged pin thread, and detailed analysis of the cracks led to identification of a primary failure mechanism called fretting-fatigue. The fretting-fatigue phenomenon is a combination of fretting wear and fatigue loading, characterized by a crack starting from the connection OD and growing towards the pipe ID.
To enhance the fatigue performance of this connection, a number of design improvements were considered. Several led to substantial modifications of the thread design. They were not implemented because the changes were estimated to have too much influence on the static behavior of the connector compared with the original qualified design basis. The focus was then put on:
- Reducing the contact pressure at the critical thread root with the aim to reduce surface damage due to fretting and hence, delay the crack initiation. One way to achieve this was to make the coupling section above the critical thread more flexible. Therefore, a specific box outer profile was designed with fatigue compliant geometric transitions, to have a thinner wall above the critical thread.
- Reducing the geometric stress concentrations at the last engaged thread root. A double radius concept was implemented in the thread profile to decrease stress concentrations “hot spots” while keeping the same static resistance.
Those modifications were optimized with Finite Element Analysis (FEA) and the improvement to fatigue performance was demonstrated with full scale tests on the Fatigue Enhanced (FE) connector design.
As described, the optimization techniques for the FE connector design were limited by a need to stay as close as possible to the original design. However, the knowledge accumulated on the failure mechanisms left the door open for more effective fatigue design improvements. Based on this and the need for an external seal for outer riser applications and higher fatigue resistance (desired target life of 200-250 years on an outer riser), it was decided to bring the connector fatigue performance to a higher level.
To reach this new level, a triangular thread profile was designed following the same principles but without the constraints to adhere to an original design. In addition, an innovative external metal-to-metal seal called “multi-seal wavelets with bending swoosh” was engineered to:
- Run without special tools during installation and make-up
- Withstand multiple make ups and break outs
- Provide true external sealing before and after fatigue cycles.
This led to a new generation of T&C riser connector combining very high fatigue resistance, easy running, multiple make and break capacity, and both internal and external sealing integrity.
As with the FE connector, the improvement in fatigue performance with the outer riser connector design was demonstrated with full-scale tests. In terms of fatigue performance, it exceeds SCF ≤ 1.3 with the DNV-B curve for the outer riser connector, based on more than 60 connectors tested on a full-scale resonant fatigue test frame.
On another hand, the large number of fatigue tests during design and qualification has accumulated a significant amount of fatigue data for different pipe ODs, wall thicknesses, and grades.
These data can be analyzed to derive the general relationship of various parameters on the fatigue performances of T&C connectors in general.
The analysis of these influencing parameters gives valuable information for the design of new T&C riser connectors. Moreover, it helps to refine the qualification programs and optimize the future testing since such analysis can back up product qualifications by interpolation and extrapolation.
Running experience
In parallel to laboratory tests, much was learned about the running aspects throughout the various rig tests and field jobs. For instance:
- High torques and power tong dies marks management
- Transition joints to specialty joints made of X-80 grade (i.e. tapered stress joints, tension joints)
- Handling and running precautions for TSA coated pipes
- Weigh compensation at make up.
Cross-section of 75⁄8-in. heavy wall HP/HT riser prototype connector.
As results of the specific riser projects run with T&C riser connectors, other lessons also have been learned to optimize their integration and installation into the riser system. The examples given above illustrate the experience gained on running of T&C connectors for riser applications.
As of today, T&C riser connectors have been run for drilling and production riser applications (both inner and outer) in the GoM, Indonesia, Brazil, West Africa, Azerbaijan, and Egypt.
Note: Taken from DOT 2008 paper.
